This was posted in MBWorld and someone directed me to contact Sixto. He and Engtwork gave me lots of help.
1991 350SD engine problem, need help!!!
Overheating problem was solved but Coolant starts to leak into the engine again. A qualified MB mechanic opened the head and found out problem is the block. It will be too expensive to take the block out, take it to a machine shop to level it ( I believe the mechanic said because of overheat, the sixth one is dented,the machine shop will wield something on it then shave it to the same level). I really don't understand how the machine shop do but I do understand that the block and the head must be level and smooth in order to put the gasket between, then the coolant will not spill into the engine.
Now, please help me out. If this method is too expensive, can I just buy an old engine from the junk yard and replace this one? If I can find 350 SD that will be great, but Benz only produce limited amount of 350SD. If I cannot find the same engine, what other engine can I use that doesn't require too much work? I meant what model and what year engine is compatible? Can someone help me?

Now I'd decided to buy a used engine from the junkyard to put in my car. I'd called many junkyard in the tristate area but no much luck. Then one place told me that they have one 1986 ( what I asked for) just came in two days ago and the engine is still in the car. 120,000 miles. I jumped up and drove one and half hour (New York traffic) to see. They jump started the car for me, the engine seems to be running fine, the mileage is 168000 not 120000 which I really don't mind. But when I looked at the car again, I have a funny feeling that it looks like 83-85 model not 1986 model. Does anyone know how the junkyard keep their record? How accurate is that? If they don't know the previous owner how do they know exactly which year is the car? According to Sixto, only 86-87 engine is 603 and can fit into my car with some adjustment. So I have to make sure that I am buying the right engine. By the way, at the back of the car it has "300SD" on one side and "turbo diesel" on the other side. Any help is really appreciated...